Ceramides are specific sphingolipids which exist in intercellular lipid lamellae of horny layers. Ceramides are known to function, in skin epidermises, to retain moisture and as a barrier against water penetration. Various ceramides have been isolated from horny layers of skins of mammals such as human, horse, bovine, and swine and from brains and other organs of such mammals, and identified. It has been reported that various kinds of ceramides exist which are roughly classified under Types 1 to 6 (see D. T. Downing et al., J. Invest. Dermatol., vol. 84, p. 410 (1985)).
The ceramides each is constituted of a sphingosine base moiety and a fatty acid moiety bonded thereto through an amide bond. Among these, the ceramides classified under Types 4 to 6 have a hydroxyl group in the fatty acid moiety in the position adjacent to the carbonyl group. Namely, this hydroxy acid having the hydroxyl group constitutes an .alpha.-hydroxy fatty acid (2-hydroxy fatty acid). It is also known that in each of these ceramides, the sphingosine base moiety is an optically active part of the (2S,3R) configuration, while the 2-hydroxy fatty acid moiety is an optically active part of the (R) configuration.
As described above, such 2-hydroxy fatty acid moieties having the hydroxyl group therein, including the optically active moieties described above, have been known. On the other hand, with respect to 3-hydroxy fatty acid moieties having the hydroxyl group therein, racemic 3-hydroxyvaleric acid is given in JP-A-2-200691 as an example of lower saturated acids from which N-acyl groups are to be formed. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, there is no description therein concerning a specific process for producing a ceramide using an optically active 3-hydroxy acid or concerning physicochemical data for such a ceramide.